[Bug c++/88509] New: Missing optimization of tls initialization

[rafael at espindo dot la]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88509

            Bug ID: 88509
           Summary: Missing optimization of tls initialization
           Product: gcc
           Version: 8.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rafael at espindo dot la
  Target Milestone: ---

Given

struct foo {
  foo();
};
static thread_local foo bar;
foo *f() { return &bar; }
foo *g() {
  static thread_local foo *bar_ptr;
  if (bar_ptr == nullptr) {
    [&]() { bar_ptr = &bar; }();
  }
  return bar_ptr;
}

GCC has to make sure bar is only initialized once. For the function f it
produces

        pushq   %rbx
        cmpb    $0, %fs:__tls_guard@tpoff
        movq    %fs:0, %rbx
        je      .L6
        leaq    _ZL3bar@tpoff(%rbx), %rax
        popq    %rbx
        ret
.L6:
       <do_the_initialization>

for g, the common code path is somewhat simpler:

        movq    %fs:_ZZ1gvE7bar_ptr@tpoff, %rax
        testq   %rax, %rax
        je      .L15
        ret
.L15:
       <do_the_initialization>


The optimization is to use the a pointer to the object as a guard instead of
using a boolean. As far as I can tell this can be applied to any static tls
variable (where the ABI is not a problem).